Loud-speaker telephone intercommunication system



March 20, 1951 E 2,545,476

LOUD-SPEAKER TELEPHONE INTERCOMMUNICATION SYSTEM Filed May 21, 1948 2 Sheets-Sheet 1 March 20, 1951 J. LEVY 2,545,476

LOUD-SPEAKER TELEPHONE INTERCOMMUNICATION SYSTEM Fi led May 21, 1948 2 Sheets-Sheet 2 Patented Mar. 20, 1951 ,LQUDeSBEAKER [TELEPHONE INTEBCQM- MUNIQATIONSYSTEM Jacques Lvy, Paris, France," now by cha'nge of name, J acqucs Lory Application May 21,1948, Serial No. 28,445

, In'Fmm May 2 The inven i n has or t bj ime 7' oud sp er telephon in a l ti ns hav ng two distinct lines, and it is characterized bythe tact that there is automatic blocking of the line not used for transmission whilst leaving tree the line used ior transmissionwhen one of two-persons in communication takes over the speaking. This is done in such manner as to avoid the Larsen effect if there is used at each station a separate microphone and loud speaker'and so as to avoid confusion or jamming if a single apparatus working alternatively as transmitter and loud speaker is used. Thisresult is attained by disposing in each line 'a two-valve amplifier, one valve of which is normally 'blocked by overpolarization and is unblocked when the corresponding line is used, the valve not blocked becoming blocked by over polarization when the other line symmetric installation. Figure .1 illustrates an installation wherein each station comprises a microphone and a loud speaker.

Figure 2 illustrates an installation wherein ach s i n comprises an appa atus working alternatively as transmitter and receiver.

The advantage of a known installation of this type is that each party canbe heard by'the other by loud speaker without having to speak in close proximity to the microphone. This result is obtained owing to an amplifier arran emen included in each .of the lines connecting the two stations.

However, in the case voi an installation comprising at each station a microphone and aloud speaker, the amplification is limited by the starting of oscillations of audible frequency known as the Larsen effect. In the, case of an installation comprising a single apparatus at each station, the Larsen effect is, not'tobe feared but it is necessary to .block the linenot. used and to free the line used and this must .be done automatically when either one tiestakes over the conversation.

It is necessary thereforein both cases to .block the line which for the time being is not .used .for

the conversation.

Accordingto the invention, in thencase cf Figure '1 this result is obtained in .the following manner:one of the amplifier valves. of the .unused lineis -blocked by an over polarization (cutoff) due to a more intenseanodic current emitd, at h s m ment, {y one of the. ampl fie valves of the line in use." Moreover, .anoth er amplifier valv of: one or, ti e, unes. nine; 2.)? is n rma y hhs edb new generat on et amplifier of "line is obtained by the tension of the two .par-

to an 'anodic current from the blockable valve of the other line (=line l-) in such manner that this v'alve of line 2 is unblocked only when-the said blockable valve of one 'i is blocked, that is to say when one of the speakers uses line 2.

In the corresponding arrangement:

(a) The blocking of the input valve of the amplifier of line --2 is obtained ;by the tension furnished by a resistance traversed by the anodic current rectified from the output valve of the amplifier of line (12) The valve normally blocked is the output valve of the amplifier of line {2, its blocking being effected by the tension Turhished by anotherresistance traversed by the' anodic current rectified from the output valve of the amplifier of line I, the intensity of the current-lacing sufficient for producing the blocking even when the installation is not in use.

(0) The blocking of the output valve of the furnished bya resistance traversed :by the anodic current rectified from a special amplifier; connected inshunt in front of the output valve of line 2.

(d) The anodic current of the output valve of the amplifier vof line i is' rectifiediby the valve which serves to feed the installation from the Sector.

Figure 1 illustrates an installation of this 'kind. It comprises two stations A and 3 each including a microphone and a loud peaker Mi and HP! for station AyandlMZ; HP2 for station B). MI and HPZ are connected by a line (I) used in the direction of the ,arrow Fl, wherein is inserted an amplifier comprising the valves Ll, 1L2.

The anode of I L3 is connected on the I one hand tov the coupling C which supplies tension to .the control grid-of L4, and onthepth-er-hand with ;the.;g;rid, of a. valve L llhe latteris connected by a coupling 05 with a rectifier Dj at the terminals ,ofwhich {is a resistance -:R.3 having one extremity connected tohthe. grid otgthe .valve- L2.

- Similarly, oneof the terminals .of the resistance R2 is connected to the grid of thevaive. L3.

Finally, the anodic current of the valve L2 passesbyway of the self-induction S, valve v' and "secondary of atransfQrmer TE, tnto a variable resistance it If, one terminal of the "latter being earthed' an 'the other being connected toithe ems one-L4;

The flow of current in the installation is as follows:

The sector U feeds the primary of T and, by known means, puts under tension the anode of the valve L2.

The valves Ll, L3, L4, L5, are fed from T5 by known means presenting no special feature. The elimination of the Larsen eilect is obtained, ac-

cording to the invention, by the combined action of the rectifiers DI and D2 and the resistances RI, R2, R3.

The operation is as follows:-Under the influence of the anodic current from the valve L2, the resistance Rl supplies, even during rest periods, a negative tension to the grid of the valve L4 which produces an effect of over polarization (cut-off) and blocks this valve. The current .fromM cannot therefore pass to HPl, so that the sounds from the loud speaker HP2 cannot be transmitted by the microphone M2 to the loud speaker HP! and the Larsen effect is suppressed.

To this blocking there is added, when speaking before Ml, the blocking of the valve L3 due to the negative tension that the resistance R2, under the action of the rectifier Dl, supplies to the grid of L3.

It will be seen, therefore, that the valve L4 is blocked whether there is speaking before the microphone Ml or not, whilst the valve L3 is blocked only when there is speaking before lVll.

If now the person at station A becomes silent and speech is taken over by the person at station B, the modulation of M2 amplified by L3, which has now become free, arrives partly at L l and partly at L5. The valve L4 being always blocked transmits nothing, but L5 excites the rectifier D2 which gives into R3 a negative tension for producing a blocking or cut-off efiect on the valve L2. The line Ml-HP2 is therefore blocked, so that the valve L 3 can then be unblocked without risk of starting the Larsen effect.

The unblocking of L4 is produced as follows: the blocking of L2 suppresses the anodic current in El and therefore the cut-off tension applied to LA. The valves L3 and L l being new unblocked, the person at station B can speak to station A and, as L2 is blocked, the reaction of l-IPI upon Ml cannot be transmitted to III-2.

In the installation of Figure 2, each line is pro vided with an amplifier comprising two valves, one being blocked normally and the other not blocked normally. The blocked valve is unblocked when use is made of the line corresponding with the amplifier, and the non-blocked valve is blocked when the other line is used.

This action of blocking and unblocking is obtained by means identical with that already described in the preceding arrangement. That is to say, one of the amplifier valves of each line is normally blocked by an appropriate over polarization and this valve is unblocked when the corresponding line is used. 0n the contrary, the valve which normally is not blocked in each amplifier is blocked by over polarization when the other line is used.

The installation in this case is symmetrical. Each line comprises, in addition to the principal amplifier, an auxiliary amplifier, from which the anodic current, rectified by a suitable rectifier, gives: the positve alternation to that one of the valves of the amplifier of the line considered for unblocking, and the negative alternation to that valve of the other line for blocking.

It will be seen from Figure 2 that each station (station A and station 13) comprises a single apparatus l? serving both as a loud speaker and transmitter. Each apparatus is connected through a coupling (ToSo) with two lines I and 2. The line I is to be used in the direction of the arrow Fl, that is to say from A to B and the line 2 in the opposite direction F2. The two lines are symmetrical. Each comprises a principal amplifier represented here as an amplifier with two valves Ll, L2 for the line l and L3, L4 for the line 2. It will be seen immediately that when a person speaks at A, the valve Ll is modulated by its grid and transmits through T2 its modulation to the second valve L2 which, by the line I and transformer So acts upon B functioning as a receiver.

According to the invention, the valves L2, L4 are blocked by over polarization of the grid. Each station comprises an auxiliary valve (El, E2) the grid of which is connected to the anode of the corresponding entry valve (Ll, L3) of the principal amplifier. The anodic current of each of the valves El, E2 is rectified by a rectifier (I-Il, H2 and Kl, K2) the positive alternations going to the blocked valves (L2, L4) and the negative alternations to the other valves (Ll, L3).

When a person speaks at A, the valve El discharges anodic current and this has for effect to remove polarization from the grid of L2 whilst augmenting the negative polarization of the grid of L3 which then becomes over polarized. Consequently, L2 is unblocked and L3 is blocked. The line l is now free whilst the line 2 is closed.

When the person at station A ceases to speak and the person at B takes over the conversation, the contrary effect is produced, so that the line 2 becomes free whilst line l is blocked.

It will be seen therefore that owing to the provisions of this invention, the line made use of is free but the other line is blocked so that whatever may be the echoes produced in the locality where the communication is received, these cannot have a repercussion towards the transmitting station.

What I claim is:

1. In a two-way communication system, the combination comprising a pair of signal transmission lines respectively having input and output amplifying valves therein, means responsive to the output valve in one of said lines when it is not transmitting a signal for rendering the output valve in the other of aid lines inoperative, means responsive to anincreased output of the output valve in said one line upon the transmission of a signal over said one line for rendering the input valve of the other of said lines inoperative, and means responsive to operation of the input valve in said other line in response to the transmission of a signal over said other line for rendering the output valve of said one line inoperative whereby said first-named means is rendered inefiective.

2. In a two-way communication system, the combination comprising a pair of signal transmission lines, one of said lines having input and output amplifying valves therein, a first biasing circuit normally energized by the other of said lines and including a rectifier and a biasing resistor for applying a cut-ofi bias to said output valve, a second biasing circuit energized in response to the transmission of a signal over said one line and including a rectifier and biasing resistor for applying a cut-off bias to said input valve to thereby prevent operation of said one line at all times when said other line is transmitting, a third biasing circuit energized by said input valve in response to the impression of a signal on said one line and including a rectifier and a biasing resistor for applying a cut-off bias to said other line, said other line having an amplifying valve therein to which said last named cut-off bias is applied for rendering said other line inoperative and thereby said first biasing circuit ineffective to perform its normal function of applying a cut-off bias to said output valve.

3. In a two-Way communication system having a pair of separate communication lines between spaced stations, a control which comprises a first grid controlled amplifier in one of said lines, means whereby said first grid controlled amplifier is normally operable to produce a control voltage when said system is not being used to transmit sound, second andthird grid controlled amplifiers in the other of said lines, means operated by said control voltage for applying a cut-off bias to said third amplifier for preventing the transmission of sound over said other line, and means operated by said second amplifier for applying a cut-off bias to said first amplifier to remove said control voltage from said last-named means and thereby render said other line operative to transmit sound.

4. In a communication system, a transmission line having first and second stages of amplification therein, circuit means normally operative for applying a cut-off bias to said second stage to render said line normally inoperative, and means responsive to the reception of a signal at said first stage for removing the bias from said second stage to render said second stage operative to amplify the signals received by said first stage.

5. In a communication system, a transmission line having first and second stages of amplification therein, circuit means normally operative for applying a cut-off bias to said second stage to render said line normally inoperative, and a control valve having a grid connected to the output of said first stage and its anode connected to said circuit means for removing said cut-off bias from said second stage in response to a signal transmitted over said line to said first stage.

6. In a communication system, a first transmission line having at least one stage of amplification therein, a. second transmission line having at least two stages of amplification therein, a circuit operated by said first line for applying a cut-off bias to the second stage of amplification in said second line to render said second line normally inoperative, and a. control valve operated by the first stage of amplification in said second line in response to a signal being transmitted over said second line for applying a cut-off bias to the amplifying stage in said first line to render said first line, and the circuit operated thereby inoperative.

7. In a two-way communication system, the combination comprising a pair of signal transmission lines respectively having input and output amplifying valves therein, a first biasing circuit responsive to operation of the output valve in one of said lines for biasing the output valve in the other of said lines to cut-off, a second biasing circuit operated by the output valve in said one line when it is transmitting signals for biasing to cut-off the input valve in the said other line, and a third biasing circuit actuated by the input valve in said other line when a signal is impressed on said other line for biasing the output valve of said one line to cut-off" whereby said first biasing circuit is rendered ineffective.

8. In a two-way communication system, the combination comprising a pair of signal transmission lines respectively having input and output amplifying valves therein, a first biasing circuit normally operative for biasing the output valve in one of said lines to cut-off, a second biasing circuit operative in response to transmission of a signal over the other of said lines for biasing the input valve in said one line to cut-off, a third biasing circuit responsive to the impression of a signal on said one line for rendering said first biasing circuit inoperative.

9. In a two-way communication system, the combination comprising a pair of signal transmission lines, one of said lines having input and output amplifying valves therein, a first biasing circuit normally operative for biasing said output valve to cut-ofi, a second biasing circuit operative in response to transmission of a signal over the other of said lines for biasing said input valve to cut-off, a third biasing circuit responsive to the impression of a signal to be transmitted on said one line for rendering said first biasing circuit inoperative.

10. In a two-way communication system, the combination comprising a pair of signal transmission lines, one of said lines having input and output I amplifying valves connected in series therein,a first biasing circuit normally operative for biasing said output valve to cut-off, a second biasing circuit operative in response to transmission of a signal over the other of said lines for biasing said input valve to cut-off, a third biasing circuit responsive to the impression of a signal to be transmitted on said one line for rendering said first biasing circuit inoperative, and for rendering said other transmission line inoperative to transmit a signal.

11. .In a communication system of the character described, the combination comprising a signal transmission line having input and output amplifying valves therein, a first biasing circuit normally operative for biasing said output valve to cut-off, and a control circuitrendered operative by said input valve in response to the impression of a signal on the line to render said biasing circuit inoperative and thereby render said output valve operative to amplify a signal fed thereto by said input valve.

4 12. In a communication system of the character described, the combination comprising a signal transmission line having input and output amplifying valves therein, a first biasing circuit normally operative for biasing said output valve to cut-off, a control circuit rendered operative by said input valve in response to the impression of a signal on the line to render said biasing circuit inoperative and thereby render said output valve operative to amplify a signal fed thereto by said input valve, a second signal transmission line, and circuit means responsive to operation of said second line for biasing said.

input valve to cut-ofif to thereby prevent operation of said control circuit.

JACQUES LEVY.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,199,220 French Apr. 30, 1940 2,205,142 Hoard June 8, 1940 2,267,622 Mitchell Dec. 23, 1941 2,341,539 Giannini Feb. 15, 1944 2,449,344 Tschumi Sept, 14, 1948 

